FIG. 1 in the enclosed drawing shows call setup signalling in an analog telephone network. Subscriber A who is making the call (the calling subscriber) expresses his wish to start the call e.g. by lifting the receiver of his terminal equipment and by entering the telephone number of the other party of the call, that is, subscriber B (the called subscriber). The exchange of subscriber A hereby sends a Setup message (messages 11 and 12) to the exchange of subscriber B over the network in order to announce the incoming call. In the network this message is called an Initial Address Message (IAM).
When subscriber B's terminal equipment alerts, an Alerting message 13 is sent to the network which announces the alerting function. The Alerting message 14 is relayed to the exchange of subscriber A from which the sound of the alerting function is relayed further to subscriber A. If subscriber B is already busy with another call when the new call arrives, the new call will bring about a call waiting signal sound in the ongoing call, if subscriber B has activated the call waiting function. When subscriber B answers the incorming call, a Setup_complete message 15 will be sent to the network and a Setup complete message 16 further to the exchange of subscriber A to indicate setup of the connection. The exchange of subscriber A acknowledges the setup of subscriber B's connection with a Setup_complete message 17.
The acknowledgement is relayed further to subscriber B's exchange with a Setup_complete message 18.
FIG. 2 in the enclosed drawing correspondingly shows call setup signalling in an ISDN network, when there are three pieces of terminal equipment TE1, TE2 and TE3 in the extension of subscriber B. Subscriber A who starts the call sends a Setup message 21 to the exchange of subscriber A which will send a starting message over the network to the exchange of subscriber B. The exchange of subscriber B sends a Setup message 22 to subscriber B's all terminal equipment. In the example shown in FIG. 2, subscriber B's terminal equipment TE3 is busy and the call waiting function is not activated therein, so terminal equipment TE3 sends a Release complete message 23 to the network to indicate non-acceptance of the call setup. Terminal equipment TE1 and TE2 send Alerting messages 24 and 25 to the network to indicate alerting of the terminal equipment. If terminal equipment TE1 and TE2 are also busy when the new call arrives, then call waiting will be performed on the line, provided that the call waiting function has been activated for the two terminal equipment. To indicate the alerting/call waiting in subscriber B's terminal equipment an Alerting message 26 is sent to subscriber A, whereby subscriber A will hear the outgoing sound (alerting sound) of the ring. In the example shown in FIG. 2, terminal equipment TEI answers the incoming call and sends a Connect message 27 to the network.
The call waiting function ends at the same time. Information on subscriber B's answering is relayed further to subscriber A in the Connect message 28. The exchange of subscriber A acknowledges the setup of subscriber B's connection with Connect_acknowledge messages 29 and 30.
It is a problem with known call setups that when subscriber B's line is busy, subscriber A may perform call waiting only if subscriber B has activated the call waiting function. When subscriber B has activated the call waiting function, the call waiting is similar for all incoming calls irrespectively of the urgency of the call. Subscriber A may express the urgency of his matter only by waiting with the call waiting function on until subscriber B picks up the incoming call.